This invention relates to RF admittance measuring systems for monitoring the condition of materials, and more particularly, to systems of this type which are adapted for use at remote locations.
Heretofore, two-wire transmitters have been utilized to monitor various conditions at a remote location. Typically, a two-wire transmitter at a remote location is connected in series with a power supply and load at another location through two transmission wires. As the condition being monitored at the transmitter varies, the effective series resistance across the transmitter varies so as to produce a change in the current drawn by the transmitter which represents (e.g., is generally proportional to) the condition being monitored. A two-wire transmitter of this type is designed for low power consumption since the amount of power available to the transmitter from the remotely located power supply may be limited. Furthermore, certain applications may require that the two-wire transmitter be "intrinsically safe" so as to permit its use in the monitoring of conditions in an explosive environment. Under these circumstances, low energy usually associated with low power consumption becomes important so as to preclude the possibility of ignition and explosion.
Although the state of the art in two-wire transmitters is adequate for monitoring various types of conditions, the prior art technology with respect to the RF admittance measurement is deficient for two-wire transmitters for the following reasons.
When measuring the RF admittance between a probe electrode and a reference surface such as a grounded vessel, the resistance in parallel with the capacitance between the probe electrode and the grounded vessel becomes very important from a power consumption standpoint. Heretofore, it has generally been assumed that shunt resistance is sufficiently small in a sufficiently large number of applications so as to render the power provided by the 4 milliamp current in a 4-20 milliamp two-wire transmitter system insufficient to power the two-wire transmitter. In other words, the shunt resistance alone might consume more power than is available at the 4 milliamp condition leaving little or no power to operate the circuitry of the transmitter.
Moreover, in order for an admittance measurement to be accurate, reliable phase-sensitive detection must be utilized. However, such reliability usually requires a substantial source of power which is inconsistent with the low power requirements of a two-wire transmitter as discussed above and the available power because of the shunt resistance. This combination of factors imposes severe restrictions on the power which is generally considered necessary to provide a reliable RF signal from a suitable oscillator. Similar restrictions are placed on the power generally considered necessary to assure that the phase detector operates with a high degree of reliability.
Another problem which is somewhat unique to admittance measurements is the isolation of the bridge network in which the unknown admittance being measured is connected. Typically, the unknown admittance being measured is from a probe electrode to ground as disclosed in Maltby et al U.S. Pat. No. 3,781,672 and Maltby U.S. Pat. No. 3,706,980, both of which are assigned to the assignee of this invention. However, a power supply at a location remote from the bridge network as in the case of a two-wire transmitter, may not be connected to ground in a manner compatible with the bridge network. It is therefore necessary to isolate the bridge network from the bridge power supply so as to permit the bridge network to be connected to ground regardless of the power supply circuit. Moreover, if the voltage across the unknown admittance were reduced to minimize power consumption, the signal representing the unbalance of the bridge network would require amplification. Accordingly, the problem exists of providing an isolated source of power for such amplification.
Other problems exist in assuring linear and stable calibration of the admittance measuring system. It is also important to provide a system which will work with various types of probes and various lengths of cables associated with the probes without adversely affecting the admittance measurement.
To a very large degree, the above-mentioned problems are encountered when the system for monitoring the condition of materials comprises a battery-operated unit rather than a two-wire transmitter. Under these circumstances, the available power is again limited.